This invention relates generally to reducing the emission of vapor phase mercury in flue gas emissions, thereby restoring air quality and enhancing the environment, and more specifically relates to minimizing the re-emission of mercury from a wet flue gas desulfurization (“FGD”) system through the addition of additives during desulfurization.
Several industrial processes, including the conversion of coal to power, include scrubbers for removal of acid gases, such as hydrochloric acid and sulfur dioxide. Hydrochloric acid is typically removed by dissolution in water and the resulting liquor is then neutralized with a substance such as lime. Sulfur dioxide is typically removed with a wet flue gas desulfurization scrubber, wherein the flue gas containing sulfur dioxide is placed in contact with water containing an alkaline material, such as limestone, lime, magnesium compounds or sodium compounds. In the scrubber, the alkaline material reacts with the sulfur dioxide to form a neutral compound such as calcium sulfate dehydrate (i.e. gypsum).
In some feed stocks, such as coal, mercury is present in small quantities. The mercury is oxidized to varying degrees when the coal is combusted, or it can be oxidized in separate unit operations designed for that purpose or for the removal of other pollutants. The portion of the mercury that remains as elemental mercury (Hg0) is gaseous and is water insoluble and will, therefore, pass through the scrubber without being affected. There are a number of methods known in the art for removing elemental mercury from flue gas.
The portion of the mercury that is oxidized is water soluble (e.g. ionic mercury) and will be absorbed into the scrubber solution. The oxidized mercury will leave the scrubber with the scrubber solution by an outlet liquid or solid stream. If the mercury stays in its oxidized, water-soluble form it can be removed by the flue gas desulfurization water slurry, commonly referred to as the “FGD liquor”, is discharged into the environment. Unfortunately, the oxidized mercury is sometimes reduced back to its elemental form, vaporized, and released into the atmosphere as part of the scrubbed flue gas in a process referred to as “mercury re-emissions.”
Several strategies are currently being developed to control the re-emission of mercury, many involving the addition of proprietary chemicals to the FGD liquor intended to either keep the mercury in its soluble form or precipitate it as a solid. Other control strategies involve the use of materials to adsorb mercury (e.g. activated carbon or gold).